Intermediate-Temperature Solid Oxide Electrolysis Cells: Development, Testing, and Characterization

The integration of renewable energy sources into the power grid necessitates advanced energy storage solutions to manage the variability of wind and solar power. Solid oxide cells (SOCs), which can function as solid oxide electrolysis cells (SOECs) to convert steam and/or CO₂ into hydrogen and/or carbon monoxide using renewable electricity, or as solid oxide fuel cells (SOFCs) to generate electricity from these gases, represent some of the most promising technologies for energy conversion and storage. They offer high conversion efficiency and provide an environmentally friendly method for producing green hydrogen. Furthermore, SOCs can operate reversibly, switching between modes to balance energy supply and demand, thereby enhancing grid stability and efficiency. However, the commercialization of SOCs is hindered by their high costs and susceptibility to degradation, particularly during prolonged electrolysis operations and at high current densities. Specialized ferritic stainless steels, such as Crofer, which are used for interconnects, provide improved corrosion resistance at elevated temperatures but account for over 50% of the total material costs in an SOC stack [1].

To enhance the economic viability of SOEC stacks, it is essential to utilize low-cost materials, particularly stainless steel. This can be achieved by reducing the operating temperature, which allows for the use of less expensive, albeit lower-grade, stainless steels as interconnects. Additionally, operating at lower temperatures helps mitigate thermally induced degradation, especially in nickel/yttria-stabilized zirconia (Ni/YSZ)-based fuel electrodes. Consequently, lowering operating temperatures can potentially reduce costs while improving the stability and lifetime of SOECs. However, a significant drawback of this approach is the reduced electrocatalytic activity and reaction kinetics. To address this issue, nanostructured electrodes modified with electrocatalysts will be utilized to enhance cell performance, as they have been extensively studied and demonstrated to be effective in recent decades.

This thesis aims to: 1) engineer nanostructured electrodes for intermediate-temperature steam electrolysis; 2) evaluate infiltration under various typical SOC operating conditions; and 3) develop and assess protective coatings for cost-effective interconnects.

The following aspects are included in this thesis:

• Utilizing infiltration to incorporate nano-sized electrocatalysts, specifically Gadolinium-doped Ceria (CGO), into the Ni/YSZ fuel electrode backbone aims to enhance cell performance under various SOC operating conditions. These conditions include intermediate temperature steam electrolysis, reversible operation between SOFC/SOEC modes, as well as CO₂ electrolysis.
• Post-test characterization, including SEM/EDS, Raman spectroscopy, was conducted to investigate the microstructural changes within the cell during steam electrolysis at an intermediate temperature. Various degradation phenomena were observed, likely induced by a high fuel electrode overpotential, i.e. >530 mV.
• The effects of protective coatings on AISI 441 steel—a more economical yet lowerquality material for interconnects—were investigated. Various parameters affecting coating quality, including coating materials, deposition techniques, and heat treatment conditions, were explored.

Regarding SOECs: Various degradation phenomena were observed after steam electrolysis during intermediate temperature operations. These included poisoning by silicon-containing species, detachment of Ni from YSZ backbone, and the formation of monoclinic zirconia, among others. The infiltration has proven effective in mitigating these negative impacts. Implementing reversible operation should be considered as a strategy to reduce the accumulation of cathodic overpotential during continuous electrolysis. However, infiltration must be optimized under different operating conditions.

Regarding interconnects: Bare AISI 441 interconnects demonstrate a low area-specific resistance (ASR) after 2000 hours of oxidation at 650 °C. Electroplating is a promising technique for applying protective coatings; however, the Ni layer potentially induce phase transformations within the steel matrix must be addressed. The heat treatment conditions should be tailored for different interconnects to prevent excessively high temperatures and prolonged durations, as low-grade steels are more susceptible to oxidation. From an economic perspective, lower heat treatment temperatures and fewer sintering steps are more advantageous.